Just wanted to ask, my bullmastiff bitch is currently in season and is therefore seperated from the clumber pup. She is on day 12 and the pup is interested. My question is , is this the likely cause of his loss of appetite as he has hardly eaten for the last 2 days. Drinking plenty and happy in himself, just doesn't want his dinner. Is this common?

Yes this is very common in male dogs with a bitch in season nearby -especially when the dog is young and not experienced, as they grow older they tend to not be too bothered during a season apart from during the fertile days.

Ditto, very common. :-) It may be kinder for him, if he could possibly stay with a family member or a good friend for a week so that he does not loose too much weight, a pup in particular will loose weight very quickly. Dogs may become too excited, some dogs can become very stressed and start to howl and cry, they have absolutely nothing on their minds but the chance to mate, hence not eating. See how he goes but put his health first and send him away if he still won't eat after tomorrow, you may find he will eat fresh meat/chicken from your hand still though. :-)